Free piston Stirling machines typically comprise a piston and a displacer which are slidably mounted within a cylinder formed in a housing. The piston is drivingly linked to either a load or a driving mechanism.
Centering of the piston is required in free piston Stirling machines due to the unequal pressure forces acting on the piston in each half cycle of the piston's reciprocation. These unequal pressure forces cause the piston to gradually creep in one direction reducing the efficiency of the machine and possibly causing the piston to collide with another part of the machine.
Conventional means for piston centering consist of centerporting which causes the gas pressures on opposite ends of the piston to equalize at a predetermined position in the piston's reciprocation cycle. However, centerporting reduces the efficiency of the machine, and designing and making the centerporting apparatus increases the manufacturing expense of the machine.
Limiting the amplitude of the piston in a free piston Stirling machine is also desirable. If, for example, a load which is being driven by a Stirling engine at a steady state suddenly decreases, and therefore less power is required from the Stirling engine, the power output of the Stirling engine must be made to match that of the decreased load. If it is not, the energy which would have been coupled through the piston to the load is coupled to the piston only, causing the piston amplitude to increase and making collision of the piston with other engine parts likely.
Conventional methods for controlling Stirling engine power output include varying the thermal energy input to the engine. A variation in the thermal energy input to the Stirling engine usually requires too much time to have a significant effect on the engine before damage occurs thus reducing its effectiveness as a short term power control method.
Therefore, the need exists for a centering apparatus for a Stirling cycle thermomechanical transducer which does not significantly reduce efficiency and which can also be arranged to limit the increasing amplitude of the piston in the event of a reduced load.